Sewage treatment



SEWAGE TREATMENT Filed Sept. l5, 1949 ATTORNEY Patented May 25, 1954 SEWAGE TREATMENT Arthur M. Kivari, Los An Millbrae, Calif., assign Incorporated, San F ration of California geles, and Earl M. Kelly, ors to Process Engineers rancisco, Calif., a corpo- Application September 15, 1949, Serial No. 115,853

8 Claims.

l This invention is concerned with treatment of sewage, i. e., water containing domestic, industrial or farm wastes having dissolved or suspended organic components. It provides improvements in methods and apparatus for clarication and purification of sewage, to the end that throughput is increased without sacrifice v inthe quality of the operation and in many instances with an actual increase in the degree of removal of both dissolved and suspended constituents.

In accordance'with the invention, theA incoming stream of sewageis mixed, while confined, with a relatively small stream of charged water. rhe latter is obtained by subjecting a stream of water to pressure, say by means of a pump, and then suddenly releasing it in the form of a jet (preferably a turbulent jet) in the presence of the gas (say air or carbon dioxide) with which the water. is to be charged. The gas is entrapped, dispersed, and at least partly dissolved in the water, thus charging it. The charged stream in confined condition is then introduced into the confined stream of sewage, so that the two blend. The resultant mixture is then discharged into a pool of liquid having an unconned upper surface i. e., ar free surface at substantially atmospheric pressure. The finely divided gas in the mixture escapes in the pool line bubbles to which oatable solids and semi-solids are attached, and rise upward to form a frothor scum on the pool surface. This froth or float is skimmed off the pool. At the Sametime heavier solids settle to the bottom. of the pool toA be removed as sludge, and the skimmed clarified liquid eiiluent is permitted to flow out from an upper level in the pool.

It is desirable to entrapthe gas in the water stream by forcing the water through an eductor or the like having a side leg through which the gas is sucked into the stream. The well known Schutte-Koerting typel of eductor which produces a turbulent'jet is preferred, because the turbulence aids in obtaining dispersal and solution of the gas in thewater.

Almost any available gas may be employed tocharge the water stream. Air serves well and has the advantage of being available always. Carbon dioxide is desirable because of its high solubility in water. Thus flue gases or any others having a high carbon dioxide content may be employed.

Preferably the conned mixture of charged water and sewage is held in'this state for a short period to condition it, i. e., to assure'that the gaseontained'inthe mixture is thoroughly: dis- 'i stantially saturated with tributed therein and also to give an opportunity for loatable material to contact the gas. The desired conditioning time may be gained'by requiring the mixture to pass upward through a conduit of substantial volume prior to discharge into the pool. Thus the conduit or column may be disposed within the pool itself with itsdischarge near and preferably just below the normal surface thereof. A columnseveral feet' in height, say l0, is recommended We have found that gas admitted into a sewage stream as described above, is particularly effective in floating solids and semisolids out of the sewage. Moreover, the operation is cheap and simple. A gas-charged stream which is only a-small fraction ofthe sewage stream itself, say idf/f@ or less, serves to oat a` large proportion-r of material out of the sewage. 'I'hefeost of" raising the pressure or" this small water Astream in order that it may entrap the gas is low (in terms of equipmentpower consumption, maintenance, etc.) as compared with the cost of raising the pressurev on the whole sewage stream to permit it to entrap thegas directly. And, since the water which is charged is ordinarily substantially free Vofr solids'etc., theopportunities for clogging pumps, eductors, etc., employed in the charging operation, are minimized.

The water which is charged may be fresh water, but preferably is clarified effluent from some stage in thesewage treatment operation. 'Ihe use of clarified elliuent improves the eiliciency of thesewage treatmentprocess by reducing dilution. Moreover, such eluent' is subgas, so that less gas is required to recharge it.

The invention has the marked advantage that existing sewage treatment plants are easily adapted to its practice. For example, a conventional clarirler provided with raking mechanisms to move settled sludge to an underflow well, may serve to contain the pool in which the flotation occurs. For optimum results however, the invention` should be practiced in equipment such as is set forth in the following detailed description' and illustrated in the accompanying drawings in which:

Fig. 1 is a now sheet illustrating schematically one presently preferredembodiment of the invention, and

Fig. 2 is another flow sheet showing schematically another embodiment of the invention.

In Fig. l, a streamr of raw sewage flowing in a pipe lil: is mixed with a side'stream of charged wateradmitted through a pipe l l. The resulting mixture of charged water and sewage is forced into the bottom of a central well I2 of a primary clarifier I3 in which both flotation and settlement occur. The clarifier is a relatively wide cylindrical tank having a gently sloping inverted conical bottom which terminates in a sludge pit I4 from which settled sludge flows through a pipe I5 to disposal. The clarifier has a conventional peripheral launder I5 around its upper edge into which clarified eiiiuent fiows. The central well rises upward in the clarier and is provided with a number of vertical slots or discharge ports I'I which communicate with the pool (retained in the tank) just below the pool level I8. A conventional rake mechanism I9 is provided in the bottom of the tank and extends from the outside wall of the tank to the central well. The rake mechanism rotates slowly about the vertical axis of the tank and the central Well and the rakes move settled sludge in spiral paths toward and into the sludge pit. If desired, the rake mechanism may be attached rigidly to the central Well structure, which is itself rotated in order to rotate the rake arms. A horizontal skim plate Eil is dosposed to skim the surface of the pool. It too is rotated slowly on the vertical axis of the tank and moves scum from the surface of the pool into a scum box 22 disposed inside the tank near the outer periphery. Material skimmed into the scum box flows through a side pipe 23 out of the tank into a sump 24 from which it is drained and sent to disposal. A coaxial round vertical baiiie plate 25 is disposed inside but slightly spaced from the tank wall and rises from a level inside the pool to a level above the pool surface. This prevents scum from overflowing with the clarified eiiiuent, which rises in the annular gap between the baiiie and the wall.

The eiuent from the primary treatment unit flows into the central well of a secondary treatment unit. This unit, like the first, is a cylindricall tank 2B with an inverted conical bottom sloping from the tank wall to a sludge pit 21. It has an upper peripheral launder 28 into which effluent flows over the tank wall, and a conventional rake mechanism 29 which, as it rotates, moves settled sludge off the entire bottom to the sludge pit. Unlike the primary unit however, the upright concentric annular bafe 30 of the secondary unit is disposed about midway between the central well and the tank wall and extends from a level just abovethe arms of the rake mechanism to a level above that of the pool in the tank. The rake arms are fastened to the central well or sha-ft 3l which rests on a sealed annular bearing 32 below the tank bottom. The shaft has a ring gear 33 around it at the top and is driven through gearing by a motor 34 resting on beams 35 overlying the pool. A blower 3B supplies low pressure air to a pipe 3l that extends down into the shaft and terminates in a porous diffuser 38. The shaft has vertical inlet ports or slots 39 in it adjacent the tank bottom and below the diffuser. It has vertical outlet ports or slots 4I) which communicate with the pool in the tank adjacent its surface.

Clarified effluent from the secondary unit is sent, in the main to disposal. A small fraction of this efliuent, say to 5% or even less, iiows to a centrifugal pump Il! through a valved line 4 IA which raises its pressure and forces it through an eductor 42, which may be of the Venturi tube or any other conventional type. The eductor has a side inlet pipe into which air is sucked as a rigidly result of the jet action developed in the eductor. It is the latter which supplies the gas-charged water for admixture with the incoming sewage, as described at the outset..

If desired, the water to the pump may be clarined effluent from the first clarifier unit, as indicated by the valved flow line 43. It may also be fresh water introduced to the pump through a valved side pipe 44.

In some instances it may be desirable to add a coagulant 45, such as alum, to the effluent of the primary unit just prior to its introduction into the secondary unit.

The sludge from the pit of the secondary unit may be sent to disposal, or part or all of it may be returned, as shown, through a valved line 46 to the sewage stream ahead of the primary unit, and preferably ahead of the point at which the charged water is introduced.

The operation of the apparatus of Fig. l is as follows:

The raw sewage to be treated is pumped or fed by gravity through the pipe which enters the bottom of the feed well in the primary clarifier. A stream of returned sludge from the secondary clarifier enters the raw feed stream ahead of this point and the charged water stream enters the influent stream between the point of entrs1 of the returned sludge and the base of the central well in the primary clarifier. The gas contained in the charged stream mixes with the influent and the returned sludge and the resultant gasified mixture is conditioned as it rises through the feed well. The gasied stream is discharged from the feed Well into the pool in the primary clarifier with resultant release of fine gas bubbles having oatable solids and semi-solids adhering thereto. The result is the formation of a copious scum which is skimmed off the top of the pool in the primary clarifier. This product is sent to disposal.

Sludge settles in the bottom of the primary clarifier and is raked into the sludge pit from whence it flows to disposal. Clarified water passes up under the annular baille in the primary clarifier, overflows the Wall into the peripheral launder and from this point the bulk of this flow and usually all of it goes to the secondary clarifier. If desired, a coagulant for the solids in the sewage is added to the efliuent from the primary clarifier just ahead of the point Where this eiuent enters the shaft of the secondary clarifier. This shaft, because of the presence of the air diffuser between the bottom slots and the top slots, acts as an air lift and at the same time supplies air to meet the biological oxygen demand of the sewage. Aerated liquid flowing up the shaft discharges into the inner pool of the secondary clarifier. Sludge settles out in the inner pool of the secondary clarifier and also in the outer annular pool. This sludge is raked to the pit of the secondary clarifier and from this point is discharged for disposal or for complete or partial return to the raw feed to the primary clarifier.

Some of the settled sludge is drawn into the bottom of the shaft through the lower ports and is recirculated through the inner pool of the secondary clarifier.

Liquid in the secondary clarifier passes underneath the intermediate annular baffle and rises upward to overflow the wall of the clarifier into the peripheral launder. The bulk of this clarified water passes out of the system. A small percentage, say 5%, may be returned to mix with menacer 5." the primaryfeeid after. charging-:Withfgass This charging: is faccomplish-edfby raisingsftheepressure off this: eiiiuentuwithi the-1 centrifugal' pump and thence passingz it: through the-.eductorarwhere xair or; otherI .suitable gas,` say: carbon .dioxfiderorI ue gas,` is .sucked iny andpartially: dissolved.

If desired;- part or yall of the Water tobezcharge'd can be: the -eiluent of the primaryrclarierwhich, as shown, may bypass the secondary clariiien and go direct. to: the= pump.

Provisionlisrmade for introducing,-freshwater y into.; the; purnpfinlet` if :.forvanyrreasonrthe supply of eiliuentifromx the clarifier operations is interrupted.

.Theiprimary -clarien which alsoeoperates' as vva flotation. machina,A isy highly eiective in1 removal off-solids,l etc.,v from the. sewagefandl consequently itfcan he very small compared vwith the secondary clariiier. Thusit needonlyrbe aboutone-lifth aslargein `plan as is the secondaryclarifier.`

If desired, the air lift andy recirculationffeature offthe secondary clariii'ermayV be emitted; al#- thoug-h these features are generally desirahle-y becazusezofl the effect ofthe airadmitted `through the; diiuser which satises the. biological oxygen v demand ,of the; material undergoingftreatment.

If desired, a'diiiuser may be placed'inthe central Wellof the primary; clariiieroflig:1` and moreover, recirculation `of.sludg`e through this well may be had byrcuttinglower.- ports inthe wallof'thewell at about: rake level.

Fig., 2 illustratesfanf arrangementV in which the prmaryclarier is equipped with azcentral'feed well' into-which1 diffused air is admitted; Referring to Fig. 2, the system comprises-aprima-ry clarierfollowed by ya secondaryoi-ariier. The secondary clarieris identica] in construction with f they secondary clarifier l inv Fig. lV and like parts are identiiied by the sainefreference characters. Ilheprimary clarier however,A although of; aboutrlthezsazne size as: the secondary cIarer, has some of thel features of tlievprimaryfclari- Iier in Fig. 1.

The primaryv clarifierv i3.y o-fFig. 2. comprisesza cylindrical tank having an' inverted conical .fbottom. 'Afcentral `shaft or feedl well' Slifextendsupl through the center of. the tankto above the liquid level therein andv is rotated by conventional means, not shown. The central shaft restsonf-an annular bearing 5i below the tank and the arms` of a` conventional `vrakefmechanisrn.- A52 v are attached toit. Itvhasfasludgelt|14' near the central shaft into which the rakes move settled sludge.

Thecenteral shaft has lower/ports 53iat;.ap proximately the level ofthe rakes and .upper ports 54 at about the leveljofthetop of thepool in thetank. A hollow porous, memberl55 which actsl asa diffuser is disposed in the central .shaft between the top and' bottom ports and air under pressure is supplied to'itbyfacentrally disposed air linefrom a blower 5B.

Theprimary clarierofFig. Zhas a skimming arm like that of the primary clarier of Fig. 1. This movesscum*over'into-ascum tank 22, from whence it iiovvs intoE an; adiacentisumpd and from this point .is1sentcto-disposal- Theanf.- nular baffle in the primary clarier of Fig. 2 is like that of the primary clarifier of Fig. 1 and is placed close to the outer wall.

The pump 4I and gas eductor 42 of the apparatus of Fig. 2 are the same as that of Fig. l and are arranged in the same fashion.

The apparatus of Fig. 2 operates exactly like that of Fig. 1 except that there is additional aeratiominitheanrimary'clarier withffcirculaticn of settled.;:sludgea;thrcwughf;thev :centraliv column;l This aids in notation and also serve :tousatisfy thebiological oxygemdemand inthe-sewage subjected to. theV process.

The?. prirriaryclarifierv in Fig.` .'2 y is of. substanf tially` the. samesizre as;:the fsecondarylclarier so as to. give'fadequatefspace inv whichxthe airs-added through the;diiuserrinpthe. primary clarifier can act uponA the :rf-:circulatingmateriall to satisfythe biologicaloxygen demand.

If desired, the primary and'. secondaryv clariners shown in Figsgl" andf2fmay be replacedtby conventional clariers; in which no aerationor circulationv occurs. Infsuchfcase; thefrecirculat ing ports, the intermediate baieandtheairdiffuserin the clariiersy are eliminated. Thiselimination somewhat reduces f the amountof solids' removed L .from the f sewage,` andgialso; reduces?. the ability of the equipment tofl satisfythe :biological oxygen demandoithesewage. However; inesome cases; this demandlmay not"behighandgmayrbe satisnedby air introduced :intothe-primaryzwclariv- 'er in' the form of` charged water.

'The primary clarier in thezapparatus '-offig; 1; and l' in fact-all-the clariers shown mayzbe provided :with air dii'fusers -r disposed outsideil the central column,- and'preferablynear'it. Thus the diluserf arrangement disclosed in coepending api-V plication Serial No. 96;*184gfled: May 31,119.49,;by Kelly andKiVari now Patent. No.y 2,568;452 may be-emp-lo-yedto provide aeration' 'in excesswf that accomplished by the-charged water;

Sewage. treatment' inraccorda-nce with ourin.-

r vention willlvary dependingupontheskindrand amount f of solidspresent.Y "Ilhe: fol-lowing; how,- ever,v is a preferred: example.r of treatment :of municipal sewage containing about. 200 to 3.00 parts perfmillion .of suspended solids; Thisfsewage r`istreated inv thevprimary Vclarierfl 3 of Fig: 1`. It is continuously introduced through" the central v'well f- I2 f of this clarifier against a' head. of about 15i`feet. The enteringy sewage is charged with -gasvintroducedz acharged water lstream which enters the incoming; stream of sewage through thej pipe I l. Forf each gallon of= sewage introduced, fromv g-to 1% gallons ofcharged waterisused. TheA cfharged water 'contains' dissolved air-'sunieient to superchargefthelentire: iniio'wingv stream lwith' from-2 to-' 3f timesthef amount of=air-which= thefsewagewill contain infsolution under atmosphere conditions-ofl temperature 'and pressure,` i. e. 1.87% byvolume.l In other-swords, the fcharged-J stream water'contains fromy 4% to-6% offair byf'volume'.

The-residence timeiin) the primary clarifier is from-5- to 30- minutes-depending upon the characteristicsof the sewage. Duringjthis timeabout 35% of the tota-lsuspended solids-'is'oate`d out and rabout-,10 settles-and is lremoved fromzthe bottomfothe clarier'assludge.' 'Thefrbalancefof the Solids is carried oiin the eilluent'to thecsece ondary clarifier, where the `greatbulkofiitsy is settled. Irrshort, the solids removal intheclarif Iierofthe invention in whi'chotation occursis from 45% -to- 55%as opposedto a solids removal of only 10% to 20% in a conventional clarifier` of the same size operating with like material and' at like throughput, but not employing otatiorr and depending upon settlement alone for solids removal. The B. O. D. reduction under the conditions of operation recited above is 25% to 30%, as compared with 10% to 20% obtained in a conventional clarier of like size operating with 'oflsewage'mixed with the theV same feed and the same throughput, but without the introduction of dissolved air. We claim:

l. In clarifying sewage, the improvement which comprises simultaneously settling and floating solids out of a pool of the sewage having an unconfined upper surface by introducing into a conned stream of the sewage under a hydraulic hea-d a smaller stream of water charged with gas containing oxygen, and introducing the resulting mixture into the pool on which the hydraulic head is less so that the gas escapes as ne bubbles in the pool and buoys up some solids to the top of the pool while other solids sink in the pool, and removing the buoyed-upl solids from the top of the pool.

2. In clarifying sewage, the improvement which comprises simultaneously settling and floating solids out of a pool of the sewage having an unconned upper surface by introducing into a confined stream of the sewage under a hydraulic head another stream of water charged with gas containing oxygen and introducing the resulting mixture into the pool where the hydraulic head is less so that the gas forms iine bubbles in the pool and buoys up some solids to the top of the pool while other solids sink in the pool, removing the buoyed-up solids from the top of the pool and removing settled solids from the bottom of the pool.

3. In clarifying sewage involving the removal of a claried liquid effluent therefrom, the improvement which comprises simultaneously settling and floating solids out o a pool of the sewage having an unconned upper surface by charging at least a portion of the removed claried eiluent with air, introducing into a confined stream of the sewage under a hydraulic head a smaller stream of the air-charged claried eiiiuent, and introducing the resulting mixture into the pool where the hydraulic head is less so that the gas forms ne bubbles in the pool and buoys up some solids to the top of the pool while other solids sink in the pool, removing the buoyed-up solids from the top of the pool and removing settled solids'from the bottom of the pool.

4. Process according to claim 3 in which the clariiied effluent is subjected to aeration and further clarification with removal of additional solids therefrom, thereafter a portion of the furr ther clarified efiiuent being air-charged and introduced to the sewage stream.

5. Process according to claim 3 in which the claried effluent is subjected to aeration and further clarification, solids therefrom as sludge, at least a portion'of the sludge being introduced into the stream of sewage sent to the pool with the air-charged clariiied effluent. i

6. In clarifying sewage, the improvement which comprises simultaneously settling and floating solids out of a pool ofthe sewage having an unconned upper surface by introducing into a coniined stream of the sewage under a hydraulic head a smaller stream of air-charged water, and introducing the resulting mixture into with removal of additional a central portion of the pool where the hydraulic head is less so that the air escapes as ne bubbles in the pool and buoys up some solids to the top of the pool while other solids sink in the pool, removing the buoyed-up solids from the central portion of the top of the pool, removing settled solids from the bottom of the pool, and removing clarified eiliuent around the upper edge of the pool.

7. In apparatus for sewage treatment including a tank for holding a pool of the sewage open to the atmosphere, means for removing sludge settling out of the sewage on the tank bottom, a bale in the tank spaced from the bottom thereof and projecting below and above the surface of the pool and dividing it into two compartments, and means for .removing claried eflluent from the upper portion of the pool in one compartment, the combination which comprises means for charging a water stream with dissolved gas containing oxygen, means for introducing a stream of the charged water into a confined stream of incoming sewage under a higher head than that existing in the pool, means for introducing the resulting mixed stream containing the dissolved gas into the pool within the other compartment, and means for removing from the surface of the pool within said other compartment solids buoyed-up by the gas given up by the contents of the pool.

6. In apparatus for sewage treatment including a tank for holding a pool of the sewage open to the atmosphere, means for removing sludge settling out of the sewage on the tank bottom, an annular baille in the tank spaced from the sides and bottom thereof and projecting below and above the surface of the pool, and means for removing claried effluent from the upper portion of the pool outside the baffle, the combination which comprises means for charging a. water stream with dissolved gas containing air, means for introducing the charged water stream under a hydraulic head into a stream of the sewage to be treated, means for introducing a stream of the resulting mixture into the pool at a lower hydraulic head within the baiile, and means for removing from the surface of the pool solids buoyed-up therein by the gas given up by the charged mixture.

References Cited in the file of this patentl UNITED STATES PATENTS Number Name .Date Re. 22,701 Kelly et al. Dec. 4, 1945 864,856 Norris Sept. 3, 1907 1,069,169 Parker Aug. 5, 1913 2,246,224 Streander June 17, 1941 2,479,403 Powers Aug. 16, 1949 2,492,486 Kivari et al Dec. 27, 1949 FOREIGN PATENTS Number Country Date 14,733 Great Britain of 1915 OTHER REFERENCES Sewerage and Sewage Treatment, by Babbitt, 6th ed. (1947), page 454. 

1. IN CLARIFYING SEWAGE, THE IMPROVEMENT WHICH COMPRISES SIMULTANEOUSLY SETTLING AND FLOATING SOLIDS OUT OF A POOL OF THE SEWAGE HAVING AN UNCONFINED UPPER SURFACE BY INTRODUCING INTO A CONFINED STREAM OF THE SEWAGE UNDER A HYDRAULIC HEAD A SMALLER STREAM OF WATER CHARGED WITH GAS CONTAINING OXYGEN, AND INTRODUCING THE RESULTING MIXTURE INTO THE POOL ON WHICH THE HYDRAULIC HEAD IS LESS SO THAT THE GAS ESCAPS AS FINE BUBBLES IN THE POOL AND BUOYS UP SOME SOLIDS TO THE TOP OF THE POOL WHILE OTHER SOLIDS SINK IN THE POOL, AND REMOVING THE BUOYED-UP SOLIDS FROM THE TOP OF THE POOL. 